Strongylus vulgaris – Bloodworm
A Dangerous Global Intestinal Parasite of Horses
Strongylus vulgaris adults live in the intestine. Larvae migrate in blood vessels. This is a deadly parasite of horses in temperate zones all over the world.
Strongylus vulgaris is a nematode (round worm) intestinal parasite of horses. Also known as blood worm, red worm, palisade worm, or just a large strongylus, it is one of the most dangerous horse parasites. An adult is actually quite small—only 14-24mm long. Like the hookworms of humans, it lives attached to the intestinal lining with a plug of intestinal tissue filling its mouth.
Epidemiology of Strongylus vulgaris
Strongylus vulgaris is found in temperate regions of the world (the zone between the tropics and the arctic) in grasslands wherever there are horses. Horses that graze together in areas where infected horses regularly graze are at greatest risk of infection. Early stages of larvae in the pasture—before they reach the infective stage—are quite susceptible to drying; therefore, a spell of hot dry weather, and breaking up of horse droppings, can greatly reduce the numbers of larvae.
Life Cycle of Strongylus vulgaris
The life cycle of S. vulgaris depends on horses grazing where droppings from infected horses contaminate the grass. Infective larvae present on vegetation in the pasture are swallowed by grazing horses. They travel to the small intestine where they shed their outer sheath, or skin.
- Larvae penetrate the intestinal lining of the horse and undergo a short period of development in the tissues there.
- Larvae migrate to arteries, particularly the mesenteric artery, where they remain, moving along the artery walls, for three to four months. They often lodge in the posterior aorta (the main vessel carrying oxygenated blood from the heart to the tissues).
- Larvae are pre-adult at about four months after infection. At this stage, they are found in nodules in the intestinal arteries.
- Pre-adults move into the intestine and attach themselves to the intestinal lining in the cecum (the first part of the large intestine after the small intestine).
- Strongylus vulgaris worms mature in the intestine after six to eight weeks. They suck blood and produce eggs that are passed in the horse’s droppings.
- Eggs deposited outside in warm moist conditions soon hatch. Larvae develop to the infective stage in one to two weeks.
Symptoms of Strongylus vulgaris Infection
Infestation with S. vulgaris is serious, and often fatal. While adult worms can cause significant blood loss in the intestine, it is the larvae that do the most serious damage. Migrating in the arteries, primarily the anterior mesenteric artery, larvae damage blood vessel walls causing inflammation and increasing the risk of blood clots and aneurisms (weak bulging vessel walls that can burst). Clots breaking away into the bloodstream can result in blockage of the artery.
Symptoms of S. vulgaris infection include:
- Fever
- Loss of appetite
- Weight loss
- Abdominal discomfort
- Constipation
- Depression
- Anemia
Antiparasitic drugs can be used to cure S. vulgaris infestations, however some drugs are only effective against adult worms, and damage to blood vessels may be permanent.
Read About Other Dangerous Nematode Parasites of Animals and Humans:
Dirofilaria immitis - Heartworm
Sources:
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
“Large Strongyles.” The Merck Veterinary Manual.
“Strongylus vulgaris in the Horse: A Review.” McCraw B M, J O Slocombe. Can Vet J. 1976 June; 17(6):150-157
